Acceleration sensor

ABSTRACT

An acceleration sensor in which a difference in resonance characteristics between two resonators can be easily adjusted even when casing components are already attached to an acceleration-sensor element includes a bimorph acceleration-sensor element having first and second resonators attached to opposite sides of a base plate with respect to a direction in which acceleration is applied. One longitudinal end or both longitudinal ends of the acceleration-sensor element is/are fixed such that the first and second resonators bend in the same direction in response to the acceleration. Changes in frequency or changes in impedance in the first and second resonators caused by the bending of the acceleration-sensor element are differentially detected in order to detect the acceleration. Opposite sides of the acceleration-sensor element with respect to the application direction of acceleration are respectively covered with a pair of casing components. Electrodes disposed on the main surfaces of the respective first and second resonators face one of opposite open planes defined by a combination of the acceleration-sensor element and the casing components with respect to a direction perpendicular to the application direction of acceleration. Accordingly, a trimming process for the electrodes can be readily performed.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to acceleration sensors, and particularly,to an acceleration sensor including a piezoelectric material.

2. Description of the Related Art

A known acceleration sensor including piezoelectric ceramics is, forexample, disclosed in Japanese Patent No. 2780594, hereinafter referredto as Patent Document 1. Such an acceleration sensor is provided with abimorph sensor element including a pair of piezoelectric units which arecomposed of piezoelectric ceramics and are integrally joined to eachother in an opposing manner. The sensor element is held inside a casingin a double-supported fashion. When acceleration is applied to theacceleration sensor, the sensor element bends, thus generating stress inthe piezoelectric units. The electric charge or voltage generated due tothe piezoelectric effect is then detected in order to determine theacceleration. Acceleration sensors of this type are advantageous in viewof their compactness and their capability of being formed easily intosurface-mounted units (chip units).

In an acceleration sensor based on the above-described principle, a biascurrent flowing from a circuit is stored in a capacitor C of thepiezoelectric material. In order to prevent the circuit from becomingsaturated, a resistor R is required for allowing the bias current to bereleased. However, since the resistor R and the capacitor C define ahigh pass filter, the acceleration in the frequencies below the cut-offlevel, such as DC and low frequency, cannot be detected.

On the other hand, an acceleration sensor disclosed in JapaneseUnexamined Patent Application Publication No. 2002-107372, hereinafterreferred to as Patent Document 2, particularly, the acceleration sensorshown in FIG. 8 in Patent Document 2, includes a single base plate whoseopposite sides respectively have first and second resonators formed of apiezoelectric material attached thereto so as to define anacceleration-sensor element, each of the first and second resonatorshaving electrodes on opposite sides thereof. One longitudinal end orboth longitudinal ends of the acceleration-sensor element is/are fixedsuch that the first and second resonators are bendable in their opposingdirection in response to acceleration. When the acceleration-sensorelement bends in response to the acceleration, changes in frequency orchanges in impedance in the first and second resonators caused by thebending of the acceleration-sensor element are differentially detectedin order to detect the acceleration.

In this case, the acceleration in a DC or low-frequency level can bedetected. Moreover, the changes in frequency or the changes in impedancein the two resonators are differentially detected instead of beingdetected in a separate manner. This counterbalances the stress (forexample, a stress caused by a change in temperature) applied to bothresonators. Thus, a high-sensitivity acceleration sensor, which isunaffected by, for example, a change in temperature, is achieved.Furthermore, because the central bending plane (i.e., a plane wherestress is 0) is set in the base plate, a large degree of tensile stressand compressive stress can be generated in the resonators disposed onthe opposite sides of the base plate. Accordingly, this improves thesensitivity of the sensor.

The two opposite sides of the acceleration-sensor element with respectto a direction in which acceleration is applied are respectively coveredwith a pair of casing components, and moreover, two opposite open planesof the combination of the acceleration-sensor element and the casingcomponents with respect to a direction perpendicular to the applicationdirection of acceleration are respectively covered with a pair of covercomponents. Accordingly, a displacement portion of theacceleration-sensor element, which is bendable in response toacceleration, is disposed within an enclosed space, whereby anacceleration sensor suitable for a surface-mounted electronic unit isachieved.

In an acceleration sensor having such a packaged structure, even if thetwo resonators attached to the opposite sides of the base plate havecompletely the same resonance characteristic, there still may be aslight difference in the resonance characteristics between the twodepending on, for example, the attachment conditions with the base plateor with the casing components. Such a difference in the resonancecharacteristics may be detected as an output signal even in a statewhere no acceleration is being applied.

For this reason, it is necessary to perform a characteristic-adjustmentprocess, such as a trimming process, in a state where the casingcomponents are attached to the acceleration-sensor element in order toprevent the difference in the resonance characteristics due to, forexample, the attachment conditions. However, in the acceleration sensordisclosed in Patent Document 2, the electrodes of the two resonatorsface the base plate or the casing components, meaning that theseelectrodes are not exposed at the exterior of the acceleration-sensorelement. Due to this reason, the trimming process cannot be performed onthe resonators in a state where the casing components are attached tothe acceleration-sensor element.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodimentsof the present invention provide an acceleration sensor in which adifference in resonance characteristics between two resonators can beeasily adjusted even when casing components are already attached to anacceleration-sensor element.

In addition, preferred embodiments of the present invention provide acompact, high-sensitivity acceleration sensor that is prevented frombeing affected by factors other than acceleration, such as a change intemperature or other factors.

According to a preferred embodiment of the present invention, anacceleration sensor includes a base plate, and first and secondresonators each formed of a piezoelectric material and each havingelectrodes on two opposite main surfaces thereof, each resonator havinga vibrating section at an intermediate portion of the resonator withrespect to the longitudinal direction thereof. The first and secondresonators are attached to opposite sides of the base plate with respectto a direction in which acceleration is applied so as to define abimorph acceleration-sensor element, wherein one longitudinal end orboth longitudinal ends of the acceleration-sensor element is/are fixedsuch that the first and second resonators bend in the same direction inresponse to the acceleration, and wherein changes in frequency orchanges in impedance in the first and second resonators caused by thebending of the acceleration-sensor element are differentially detectedin order to detect the acceleration. Opposite sides of theacceleration-sensor element with respect to the application direction ofacceleration are respectively covered with a pair of casing components.The first and second resonators are attached to the base plate such thatthe electrodes of the first and second resonators face at least one ofopposite open planes defined by a combination of the acceleration-sensorelement and the casing components with respect to a direction that issubstantially perpendicular to the application direction ofacceleration.

In a case where the acceleration-sensor element has a bimorph structurein which the resonators are attached to the opposite sides of the singlebase plate, and the central bending plane is positioned at the centralportion of the base plate with respect to the thickness of the baseplate, when acceleration is applied to the acceleration-sensor element,the base plate functions as a mass body so as to effectively generate atensile stress in one resonator and a compressive stress in the otherresonator. In this case, the frequency in the resonator with tensilestress decreases while the frequency in the resonator with compressivestress increases. By differentially detecting the changes in frequencyor the changes in impedance in the resonators, the acceleration can bedetected. Moreover, since the changes in frequency or the changes inimpedance in the two resonators are detected in a differential manner,the stress applied to both resonators (for example, a stress caused by achange in temperature) can be counterbalanced. Accordingly, ahigh-sensitivity acceleration sensor that is unaffected by, for example,a temperature change is provided.

Preferred embodiments of the present invention are arranged such thatthe opposite sides of the acceleration-sensor element with respect tothe application direction of acceleration are respectively covered witha pair of casing components, and such that the first and secondresonators are attached to the base plate in a manner such that theelectrodes of the first and second resonators face at least one of theopposite open planes defined by the combination of theacceleration-sensor element and the casing components with respect tothe direction that is substantially perpendicular to the applicationdirection of acceleration. Specifically, since the electrodes of thefirst and second resonators are exposed at the at least one open plane,the trimming process for the electrodes can be performed easily, therebysolving the problem of the difference in characteristics between the tworesonators. To prevent the difference in the resonance characteristics,a trimming process may be performed on each electrode by using, forexample, laser. Alternatively, the electrodes may be coated with, forexample, frequency-regulating ink.

There are, for example, two approaches for obtaining a signalproportional to the acceleration acting upon the acceleration-sensorelement based on the signals differentially detected from the tworesonators. One approach is to oscillate the first and second resonatorsseparately with different frequencies, determine theoscillating-frequency difference, and obtain the signal proportional tothe acceleration based on the frequency difference. The other approachis to oscillate the first and second resonators with the same frequency,detect the phase difference or the oscillation difference based on thedifference in electric impedance between the resonators, and obtain thesignal proportional to the acceleration based on the phase difference orthe oscillation difference.

Furthermore, one of the opposite open planes defined by the combinationof the acceleration-sensor element and the casing components withrespect to the direction that is substantially perpendicular to theapplication direction of acceleration is preferably provided with afirst electrode connected with one of the electrodes of the firstresonator, a second electrode connected with one of the electrodes ofthe second resonator, and a third electrode connected with the otherelectrode of the first resonator and with the other electrode of thesecond resonator.

In this case, since three electrodes are exposed at the same surface,the resonance characteristics of the resonators can be easily measuredby allowing terminals of a measuring device to come into contact withthese electrodes. This is advantageous in that the trimming process canbe readily performed.

Furthermore, the base plate and the first and second resonators arepreferably formed of at least one material having substantially the samecoefficient of thermal expansion.

If the coefficient of thermal expansion differs significantly betweenthe base plate and the first and second resonators, a tensile stress ora compressive stress may be generated in the resonators due to a changein temperature in the environment even when no acceleration is applied.This leads to changes in frequency or changes in impedance. By allowingthe base plate and the first and second resonators to have substantiallythe same coefficient of thermal expansion, the temperature drift relatedto the output from the sensor can be inhibited, thus reducing thermalhysteresis.

The base plate and the first and second resonators may be formed of thesame material, or may be formed of different materials. The coefficientof thermal expansion between the base plate and the resonators may bedifferent to an extent such that the changes in frequency or the changesin impedance in the resonators in an operating temperature limit arewithin an error range and are thus significantly small.

Furthermore, it is preferable that only one longitudinal end of theacceleration-sensor element is fixed, and that the opposite open planesdefined by the combination of the acceleration-sensor element and thecasing components with respect to the direction that is substantiallyperpendicular to the application direction of acceleration arerespectively covered with a pair of cover components such that adisplacement portion of the acceleration-sensor element, which isbendable in response to the acceleration, is disposed within an enclosedspace. Such a packaged structure allows the displacement portion to beblocked from the outside, whereby a surface-mounted unit that isprevented from being affected by, for example, moisture and dust isprovided.

Furthermore, one of the electrodes in each of the first and secondresonators is preferably disposed at a free-end side of the resonatorand is preferably connected with a common electrode via an extractionelectrode provided on the base plate, the common electrode beingprovided at a fixed-end side of an outer surface of a combination of thecasing components and the cover components. Moreover, the otherelectrode in the first resonator is preferably disposed at a base-endside of the first resonator, the electrode being connected with a firstindependent electrode provided at a free-end side of the outer surfaceof the combination of the casing components and the cover components,the electrode being connected with the first independent electrode via afirst extraction electrode provided on one of the casing components. Theother electrode in the second resonator is preferably disposed at abase-end side of the second resonator, the electrode being connectedwith a second independent electrode provided at the free-end side of theouter surface of the combination of the casing components and the covercomponents, the electrode being connected with the second independentelectrode via a second extraction electrode provided on the other casingcomponent.

When using an acceleration-sensor element of a cantilever structure,three electrodes are concentrated at the base-end portion of theacceleration-sensor element, and for this reason, it is difficult to setthese electrodes distant from one another on the outer surface of thepackage. In order to set the three external electrodes distant from oneanother, one pair of the electrodes from the two resonators is connectedto the common electrode, provided at the fixed-end side of the outersurface of the package (the combination of the casing components and thecover components), via the base plate, and the other pair of the tworemaining electrodes is respectively connected to two independentelectrodes, provided at a side of the outer surface opposite to thefixed-end side of the package, namely, the free-end side, via the casingcomponents. Accordingly, when used as a surface-mounted unit, a shortcircuit is prevented from occurring among the electrodes.

Furthermore, a height of the first and second resonators in a directionthat is substantially perpendicular to the application direction ofacceleration is preferably smaller than a height of the base plate inthe direction that is substantially perpendicular to the applicationdirection of acceleration.

By allowing the first and second resonators to have a smaller heightthan the base plate in the direction that is substantially perpendicularto the application direction of acceleration, the cross-sectional areaof the first and second resonators can be reduced. This increases thetensile stress and the compressive stress generated in the resonators inresponse to acceleration, thus improving the sensitivity (S/N ratio).

Furthermore, the first and second resonators are preferably attached tothe opposite sides of the base plate at positions where the first andsecond resonators are opposed to each other.

Although it is possible to attach the two resonators to the oppositesides of the base plate at positions where the two resonators do notoppose each other, such a structure may lead to detection errors. Indetail, this is due to the fact that if the acceleration-sensor elementbends in response to an external force from a direction other than theapplication direction of acceleration (off-axis bending), the tworesonators may generate different signals. In contrast, by attaching thetwo resonators to the opposite sides of the base plate at positionswhere the two resonators are opposed to each other, signals can bedetected from the two resonators in a differential manner. Thus, thedifference in detection with respect to the off-axis bending can becompensated for.

Furthermore, each of the first and second resonators is preferablyattached to a central portion of the base plate with respect to a heightdirection of the base plate, the height direction being substantiallyperpendicular to the application direction of acceleration.

Consequently, in addition to being attached to the opposite sides of thebase plate at positions where the two resonators are opposed to eachother, each resonator may be attached to the central portion of the baseplate with respect to the height direction. This structure can furthercompensate for the difference in detection since no stress acts upon thetwo resonators in response to off-axis bending.

Accordingly, preferred embodiments of the present invention provide anacceleration-sensor element having a bimorph structure in whichresonators are attached to opposite sides of a base plate. Whenacceleration is applied to the acceleration-sensor element, changes infrequency or changes in impedance in the resonators are detected in adifferential manner. Accordingly, a high-sensitivity acceleration sensorthat is unaffected by, for example, a temperature change is provided.

Moreover, since the first and second resonators are attached to the baseplate such that at least one of the electrodes on the corresponding mainsurface of each resonator faces one of opposite open planes defined by acombination of the acceleration-sensor element and the casing componentswith respect to a direction that is substantially perpendicular to theapplication direction of acceleration, an adjustment process forreducing the difference in characteristics between the two resonatorscan be readily performed. This solves the problem of the difference incharacteristics between the two resonators. As a result, output signalsare prevented from being generated when no acceleration is beingapplied, thus achieving less detection errors.

These and other features, elements, characteristics and advantages ofthe present invention will become more apparent from the followingdetailed description of preferred embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of an acceleration sensoraccording to a first preferred embodiment of the present invention.

FIG. 2 is an exploded perspective view of the acceleration sensor shownin FIG. 1.

FIG. 3 is an exploded perspective view of an acceleration-sensor elementprovided in the acceleration sensor shown in FIG. 1.

FIG. 4 is a plan view of the acceleration sensor shown in FIG. 1 in astate where cover components of the acceleration sensor are removed.

FIG. 5 includes perspective views illustrating a method for cutting amaster substrate into segments in order to form resonators.

FIG. 6 is a circuit diagram of an example of an acceleration sensordevice provided with the acceleration sensor according to a preferredembodiment of the present invention.

FIG. 7 is circuit diagram of another example of an acceleration sensordevice provided with the acceleration sensor according to a preferredembodiment of the present invention.

FIG. 8 is an exploded perspective view of an acceleration sensoraccording to a second preferred embodiment of the present invention.

FIG. 9 is an exploded perspective view of the acceleration sensor shownin FIG. 8 in a state where cover components of the acceleration sensorare removed.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be described.

First Preferred Embodiment

FIGS. 1 to 5 illustrate an acceleration sensor according to a firstpreferred embodiment of the present invention.

An acceleration sensor 1A includes a bimorph acceleration-sensor element2A supported in a cantilever manner by a pair of insulative casingcomponents 6 and a pair of insulative cover components 7 composed of,for example, insulative ceramics. Referring to FIGS. 2 and 3, if thedirection in which acceleration G is applied is defined as the y-axisdirection, the longitudinal and height directions of theacceleration-sensor element 2A are defined as the x-axis direction andthe z-axis direction, respectively.

The acceleration-sensor element 2A in the first preferred embodimentincludes resonators 3 and 4 which are integrally attached to tworespective opposite sides of a base plate 5 with respect to theapplication direction of acceleration (y-axis direction) viacorresponding spacers 51 to 54. The resonators 3 and 4 are resonators ofan energy-trap thickness-shear vibration type and each include apiezoelectric ceramic plate strip. The resonators 3 and 4 respectivelyinclude a pair of electrodes 3 a and 3 b and a pair of electrodes 4 aand 4 b. The electrodes 3 a and 3 b are respectively disposed on upperand lower main surfaces of the piezoelectric ceramic plate strip of theresonator 3, and the electrodes 4 a and 4 b are respectively disposed onupper and lower main surfaces of the piezoelectric ceramic plate stripof the resonator 4. One set of the electrodes 3 a and 4 a of theresonators 3 and 4 is exposed at the upper side of theacceleration-sensor element 2A, whereas the other set of the electrodes3 b and 4 b is exposed at the lower side of the acceleration-sensorelement 2A. A first-end portion of the electrode 3 a on the uppersurface of the resonator 3 is opposed to a second-end portion of theelectrode 3 b on the lower surface at an intermediate portion of theresonator 3 with respect to the longitudinal direction thereof.Similarly, a first-end portion of the electrode 4 a on the upper surfaceof the resonator 4 is opposed to a second-end portion of the electrode 4b on the lower surface at an intermediate portion of the resonator 4with respect to the longitudinal direction thereof. On the other hand,the second-end portion of the electrode 3 a and the first-end portion ofthe electrode 3 b extend away from each other towards the opposite endsof the resonator 3, and similarly, the second-end portion of theelectrode 4 a and the first-end portion of the electrode 4 b extend awayfrom each other towards the opposite ends of the resonator 4. Theresonators 3 and 4 preferably have substantially the same height H₁ inthe z-axis direction, and the height H₁ is determined based on theresonance frequency of the resonators 3 and 4. Since the height H₁ ofthe resonators 3 and 4 is smaller than a height H₂ of the base plate 5in the z-axis direction, the stress generated in the resonators 3 and 4due to acceleration applied to the resonators 3 and 4 is greater than ina case where H₁=H₂. In the first preferred embodiment, H₁ is preferablyset at about ⅕ or less of H₂.

As shown in FIG. 5, the resonators 3 and 4 are preferably formed bycutting a single master piezoelectric substrate M into segments, andpairing adjacent cut segments so as to form pairs of resonators. Thisreduces the difference in the resonance characteristics including thetemperature characteristics between the resonators of each pair.Accordingly, the difference in the output signal between the tworesonators, which may be caused by a change in temperature, is reducedso as to achieve an acceleration sensor having less output fluctuation.

Even if the resonators 3 and 4 are a pair of adjacent segments cut fromthe same master piezoelectric substrate, there still may be cases wherethe resonance characteristics between the two resonators 3 and 4 aredifferent due to, for example, being attached to the spacers and thebase plate 5. Such different characteristics are output as an outputsignal even when no acceleration is being applied. The set of electrodes3 a and 4 a of the respective resonators 3 and 4 is exposed at one sideof the acceleration-sensor element 2A, and similarly, the set ofelectrodes 3 b and 4 b of the respective resonators 3 and 4 is exposedat the other side of the acceleration-sensor element 2A. Consequently,if the resonance characteristics between the resonators 3 and 4 aredifferent, the electrodes exposed at the upper side or the lower side ofthe acceleration-sensor element 2A may be trimmed using, for example,laser, or may be coated with, for example, frequency-regulating ink sothat the resonance characteristics can be adjusted in order to reducethe difference in the characteristics. Such a trimming process or anink-coating process is performed after an attachment process of thecasing components 6 and a fabrication process of internal electrodes 61,62 b, and 63 b (see FIG. 4). In that case, since measuring terminals cancome into contact with the three internal electrodes 61, 62 b, and 63 bdisposed on the upper surface of the casing components 6, the trimmingprocess can be performed easily while measuring the characteristics ofthe resonators 3 and 4. As a result, a high-precision accelerationsensor with less detection error can be provided.

The upper and lower main surfaces of the resonator 3 are provided withspacers 31 and 32 having the same thickness as the resonator 3. Thespacers 31 and 32 are fixed adjacent to two respective opposite ends ofthe resonator 3 with respect to the longitudinal direction of theresonator 3. Similarly, the upper and lower main surfaces of theresonator 4 are provided with spacers 41 and 42 having the samethickness as the resonator 4. The spacers 41 and 42 are fixed adjacentto two respective opposite ends of the resonator 4 with respect to thelongitudinal direction of the resonator 4. An area where the electrodes3 a and 3 b are opposed to each other and an area where the electrodes 4a and 4 b are opposed to each other define vibrating sections. Eachvibrating section is disposed where the pairs of spacers 31 and 32 orthe pairs of spacers 41 and 42 are not disposed. In the first preferredembodiment, the spacers 32 and 42 disposed adjacent to free ends of therespective resonators 3 and 4 have a greater length than the spacers 31and 41 disposed adjacent to base ends of the respective resonators 3 and4. For this reason, the vibrating sections of the resonators 3 and 4 aredisposed close to the base end, i.e. a fixed end, of theacceleration-sensor element 2A. Because a stress generated in responseto acceleration is greater towards the base end of a cantileverstructure, providing the vibrating sections close to the base ends ofthe resonators 3 and 4 allows the resonators to receive a greaterstress, thus improving the sensitivity of the sensor. The height of thecombination of the resonator 3 and the spacers 31 or 32 and the heightof the combination of the resonator 4 and the spacers 41 or 42 arepreferably substantially equal to the height H₂ of the base plate 5.

Alternatively, the spacers 31, 32, 41, and 42 may be omitted such thatthe resonators 3 and 4 are directly attached to the two respectiveopposite sides of the base plate 5.

The resonators 3 and 4 are respectively attached to positions on the twoopposite sides of the base plate 5 where the resonators 3 and 4 areopposed to each other, and particularly, are most preferably attached tothe central portions of the base plate 5 with respect to the heightdirection of the base plate 5. This is due to the fact that even if theacceleration-sensor element were to bend in response to an externalforce from a direction other than the direction in which theacceleration is applied (off-axis bending), the difference in detectionwith respect to the off-axis bending can be compensated for by receivingsignals from the two resonators 3 and 4 in a differential manner. Thedetection difference between the two resonators 3 and 4 opposed to eachother is reduced due to the fact that, even in the case of off-axisbending, the same amount of stress acts upon the two resonators. Inparticular, attaching the two resonators 3 and 4 to the centralpositions of the base plate 5 with respect to the height direction ofthe base plate 5 further reduces the detection difference. Specifically,this is due to the fact that even when stress is generated in theresonators 3 and 4 due to off-axis bending, since each of the resonators3 and 4 bends about a central bending plane with respect to the heightdirection, the stress is counterbalanced within the resonator 3 or 4.

One side surface of the combination of the resonator 3 and the spacers31 with respect to the y-axis direction is provided with a connectionelectrode 33 connected with the electrode 3 a of the resonator 3 andextending continuously across the side surface in the height direction(z-axis direction). Similarly, the other side surface of the combinationof the resonator 3 and the spacers 32 with respect to the y-axisdirection is provided with a connection electrode 34 connected with theelectrode 3 b of the resonator 3 and extending continuously across theside surface in the height direction (z-axis direction). On the otherhand, one side surface of the combination of the resonator 4 and thespacers 41 with respect to the y-axis direction is provided with aconnection electrode 43 connected with the electrode 4 a of theresonator 4 and extending continuously across the side surface in theheight direction (z-axis direction). Similarly, the other side surfaceof the combination of the resonator 4 and the spacers 42 with respect tothe y-axis direction is provided with a connection electrode 44connected with the electrode 4 b of the resonator 4 and extendingcontinuously across the side surface in the height direction (z-axisdirection). Specifically, the connection electrodes 33 and 43 disposedclose to the base ends of the resonators 3 and 4, respectively, aredisposed on the outer side surface of the combination of the resonator 3and the spacers 31 and the outer side surface of the combination of theresonator 4 and the spacers 41.

The base plate 5 is an insulative plate having the same length as theresonators 3 and 4, and is bendable with respect to a central bendingplane (indicated by a dashed line N1 in FIG. 4) in response toacceleration G applied to the acceleration-sensor element 2A. Thecentral bending plane is positioned at the central portion of the baseplate 5 with respect to the thickness direction (y-axis direction) ofthe base plate 5. The base plate 5 and each of the resonators 3 and 4have a gap 5 a therebetween (see FIG. 5) which is given a widerdimension than the range in which the resonator 3 or 4 vibrates in anenclosed manner. In the first preferred embodiment, although the spacers51 to 54 are attached to the corresponding sides of the base plate 5 andare separated by a predetermined distance in the longitudinal directionof the base plate 5 in order to form the gaps 5 a, the opposite sides ofthe base plate 5 may alternatively be provided with depressions in placeof the spacers. As a further alternative, the base plate 5 and each ofthe resonators 3 and 4 may have an adhesive layer therebetween havingenough thickness for forming gaps.

The spacers 51 and 52 disposed adjacent to the base end preferably havesubstantially the same length as the spacers 31 and 41 disposed adjacentto the base ends of the respective resonators 3 and 4. Moreover, theheight of the spacers 51 and 52 (in the z-axis direction) is preferablysubstantially the same as the height H₂ of the base plate 5. Similarly,the spacers 53 and 54 disposed adjacent to the free end have the samelength as the spacers 32 and 42 disposed adjacent to the free ends ofthe respective resonators 3 and 4. Moreover, the height of the spacers53 and 54 (in the z-axis direction) is preferably substantially the sameas the height H₂ of the base plate 5.

The resonators 3 and 4, the spacers 31, 32, 41, and 42, the base plate5, the spacers 51 to 54 define the acceleration-sensor element 2A andare composed of materials having the same coefficient of thermalexpansion as that of the resonators 3 and 4 (for example, a ceramicmaterial such as PZT). This prevents stress from being generated in theresonators 3 and 4 due to differences in thermal expansion caused by achange in temperature.

One side surface of the base plate 5 having the spacers 51 and 53attached thereon is provided with an extraction electrode 5 b extendingover the entire length of the side surface. The extraction electrode 5 bis connected with an internal electrode 61 extending continuously acrossthe top surface of the base-end portion of the acceleration-sensorelement 2A when the resonators 3 and 4 are in a combined state. On theother hand, an internal electrode 64 extends continuously across afree-end portion of the top surface of the combination of the base plate5 and the spacers 53, 54, 32, and 42. The internal electrode 64functions as a connector for interconnecting the extraction electrode 5b disposed on one side surface of the base plate 5 with the connectionelectrodes 34 and 44 disposed on the side surfaces of the respectiveresonators 3 and 4.

The two opposite sides of the acceleration-sensor element 2A withrespect to the application direction of acceleration G are respectivelycovered with the pair of left and right casing components 6. Each casingcomponent 6 is substantially U-shaped in cross section, and a projection6 a disposed adjacent to a first end of the casing component 6 isattached to the base-end portion of one of the opposite side surfaces ofthe acceleration-sensor element 2A. On the other hand, a projection 6 bdisposed adjacent to a second end of one casing component 6 is attachedto a projection 6 b of the other casing component 6 via a spacer 2 adisposed therebetween. The spacer 2 a according to the first preferredembodiment is formed by cutting a longitudinal end-segment of theacceleration-sensor element 2A, and includes portions of the base plate5, the resonators 3 and 4, and the spacers 53, 54, 32, and 42. Theprojections 6 a and 6 b of each casing component 6 have a depression 6 cdisposed therebetween, which is a space where the acceleration-sensorelement 2A is allowed to bend into. Moreover, each casing component 6 isprovided with a stopper 6 d disposed near an inner side of thesecond-end projection 6 b. The stopper 6 d restricts anover-displacement of the acceleration-sensor element 2A when a largeamount of acceleration G is applied so as to prevent theacceleration-sensor element 2A from being deformed or damaged. If thedegree of bending of the acceleration-sensor element 2A is extremelysmall and the bending spaces can thus be formed based on the thicknessof adhesive layers between the casing components 6 and theacceleration-sensor element 2A, the depressions 6 c and the stoppers 6 dmay be omitted.

The inner side surface and the top surface of one casing component 6 arerespectively provided with extraction electrodes 62 a and 62 b which areconnected with each other, and the inner side surface and the topsurface of the other casing component 6 are respectively provided withextraction electrodes 63 a and 63 b which are connected with each other.The casing components 6 are joined with the acceleration-sensor element2A via an electrically conductive adhesive for allowing the electrodes33 and 62 a to be electrically connected with each other, and theelectrodes 43 and 63 a to be electrically connected with each other. Inthis case, an anisotropic electrically-conductive adhesive is preferablyused in order to prevent a short circuit between the internal electrode61, extending continuously across the base-end portion of the topsurface of the combination of the casing components 6 and theacceleration-sensor element 2A, and an external electrode 71, andbetween the electrode 33 and the electrode 43.

The extraction electrodes 62 b and 63 b disposed on the top surfaces ofthe corresponding casing components 6 are aligned with the internalelectrode 64 disposed on the free-end portion of the top surface of theacceleration-sensor element 2A. The electrodes 62 b, 63 b, and 64 areformed after the casing components 6 are attached to theacceleration-sensor element 2A, and can be fabricated simultaneously byperforming, for example, a sputtering process or a deposition process onthe top surface of the combination of the casing components 6 and theacceleration-sensor element 2A. In this case, the internal electrode 61can also be formed at the same time.

The upper and lower open planes of the combination of theacceleration-sensor element 2A and the casing components 6 arerespectively covered with the pair of upper and lower cover components7. The inner surface of each cover component 7 is provided with acavity-forming recess 7 a for preventing the acceleration-sensor element2A from coming into contact with the cover component 7. A peripheralregion surrounding the recess 7 a is attached to one of the open planes.For this reason, a portion of the acceleration-sensor element 2A to bedisplaced in response to acceleration G is completely enclosed by thecasing components 6 and the cover components 7. Similar to the casingcomponents 6, the cavity-forming recess 7 a in the inner surface of eachcover component 7 may be omitted if the cavity can be formed based onthe thickness of an adhesive layer provided along the frame region onthe inner surface of the cover component 7.

The outer surface of each cover component 7 is provided with a portionof an external electrode 71 positioned adjacent to the base end of theacceleration-sensor element 2A, and portions of two external electrodes72 and 73 positioned close to the free end of the acceleration-sensorelement 2A. Referring to FIG. 1, the two external electrodes 72 and 73are positioned distant from the external electrode 71 in thelongitudinal direction (x-axis direction), and moreover, are disposed ontwo opposite sides from each other in the application direction ofacceleration (y-axis direction). The positioning of the two externalelectrodes 72 and 73 is not limited to that shown in FIG. 1.Alternatively, the external electrodes 72 and 73 may be disposed at anend opposite to the end at which the external electrode 71 is disposed,such that the two electrodes 72 and 73 are disposed on opposite sides inthe y-axis direction at that end.

The acceleration sensor 1A having the structure described above has thefollowing conductive path.

Specifically, the upper electrode 3 a of the resonator 3 is connectedwith the external electrode 72 via the connection electrode 33 and theextraction electrodes 62 a and 62 b. On the other hand, the upperelectrode 4 a of the resonator 4 is connected with the externalelectrode 73 via the connection electrode 43 and the extractionelectrodes 63 a and 63 b. The lower electrodes 3 b and 4 b of therespective resonators 3 and 4 are interconnected with each other via theconnection electrodes 34 and 44 and the internal electrode 64, and areconnected with the external electrode 71 via the extraction electrode 5b disposed on one side surface of the base plate 5, and the internalelectrode 61.

Although only one extraction electrode 5 b is provided on one sidesurface of the base plate 5, two extraction electrodes 5 b mayalternatively be provided on the two opposite side surfaces of the baseplate 5. This may contribute to a further prevention of disconnection ofthe conductive path.

Accordingly, a surface-mounted-chip acceleration sensor 1A is obtained.

FIG. 6 is a circuit diagram of an example of an acceleration sensordevice provided with the acceleration sensor 1A.

Such a sensor device utilizes separate oscillation effects of theacceleration-sensor element 2A. Specifically, the external electrodes 72and 71 of the acceleration sensor 1A are connected with an oscillationcircuit 9 a, and the external electrodes 73 and 71 are connected with anoscillation circuit 9 b. Each of the oscillation circuits 9 a and 9 bmay be, for example, the commonly known Colpitts oscillation circuit.The resonators 3 and 4 are separately oscillated by the respectiveoscillation circuits 9 a and 9 b. The oscillating frequencies f₁ and f₂are then input to a frequency-difference counter 9 c. Subsequently, thefrequency-difference counter 9 c outputs a signal V₀, which isproportional to the difference in the frequencies.

When acceleration G is applied to the acceleration sensor 1A, an inertiaforce acts upon the acceleration-sensor element 2A in a directionopposite to the direction in which the acceleration is applied. Thisbends the acceleration-sensor element 2A in the opposite direction tothe application direction of acceleration G. The bending of theacceleration-sensor element 2A generates stress, thus producing tensilestress in one of the resonators and compressive stress in the otherresonator. In the case where resonators of a thickness-shear vibrationtype are used, the oscillating frequency of the resonator with tensilestress decreases, whereas the oscillating frequency of the resonatorwith compressive stress increases. Accordingly, the difference in thefrequencies is obtained via the external electrodes 71, 72, and 73 sothat the signal V₀ proportional to the acceleration G can be obtained.

Using the acceleration sensor 1A in an environment where there is achange in temperature may lead to thermal expansion of the resonators 3and 4, the base plate 5, the casing components 6, and the covercomponents 7. If the coefficient of thermal expansion is different amongthe resonators 3 and 4 and the base plate 5, the acceleration-sensorelement 2A may bend when the temperature changes, thus generating stressin the resonators 3 and 4. This means that the difference in thefrequencies may change due to factors other than acceleration. On theother hand, if the resonators 3 and 4 and the base plate 5 are composedof materials having substantially the same coefficient of thermalexpansion, the same amount of stress will be generated in response to achange in temperature. Consequently, the outputs from the two resonators3 and 4 are received by the frequency-difference counter 9 c in adifferential manner, such that the changes in the output signals causedby, for example, a change in temperature affecting both resonators 3 and4 can be counterbalanced. Accordingly, an acceleration sensor devicehaving sensitivity that reacts only to acceleration G can be obtained.

On other hand, even if the coefficient of thermal expansion among theacceleration-sensor element 2A, the casing components 6, and the covercomponents 7 is different, a temperature change simply does not lead toa generation of stress in the acceleration-sensor element 2A since theacceleration-sensor element 2A is supported by these components only ina cantilever manner.

FIG. 7 illustrates another example of an acceleration sensor deviceprovided with the acceleration sensor 1A.

This acceleration sensor device utilizes a single oscillation effect ofthe acceleration-sensor element 2A. Specifically, the externalelectrodes 72 and 73 of the acceleration sensor 1A are connected with adifferential impedance sensor circuit 9 d, and the external electrode71, which is a common electrode, is connected with an oscillationcircuit 9 e. Moreover, reference numerals 9 f and 9 g indicate matchingresistors. In this case, both resonators 3 and 4 are oscillated with thesame frequency by the oscillation circuit 9 e, so that the phasedifference or the oscillation difference can be detected based on thedifference in electrical impedance between the resonators 3 and 4. Thus,the output V₀ proportional to acceleration G is obtained via thedifferential impedance sensor circuit 9 d. In order to achieveoscillation with the same frequency, the oscillator circuit 9 e may beformed in view of feedback on an output from one of the resonators or acombination of outputs from both resonators.

In this case, like the example shown in FIG. 6, a signal proportional toacceleration G can be obtained while also counterbalancing the changesin the outputs caused by, for example, a change in temperature.Accordingly, an acceleration sensor device having sensitivity solelyagainst acceleration G can be obtained.

Second Preferred Embodiment

FIGS. 8 and 9 illustrate an acceleration sensor according to a secondpreferred embodiment.

An acceleration sensor 1B includes a bimorph acceleration-sensor element2B held and enclosed by the casing components 6 and the cover components7, which are formed of, for example, insulative ceramics, in adouble-supported fashion. Components equivalent to those in the firstpreferred embodiment shown in FIGS. 1 to 4 are indicated by the samereference numerals, and descriptions of those components will thus beomitted.

The two longitudinal ends of the acceleration-sensor element 2B arefixedly supported by the pair of casing components 6 such that theacceleration-sensor element 2B horizontally intervenes the casingcomponents 6. The casing components 6 are substantially U-shaped incross section. Furthermore, the cover components 7 are respectivelyattached to the upper and lower open planes.

The upper electrodes 3 a and 4 a of the resonators 3 and 4 arerespectively connected with internal electrodes 61 a and 61 b, which aredisposed on first end-portions of the top surfaces of the casingcomponents 6, via the connection electrodes 33 and 43. On the otherhand, the lower electrodes 3 b and 4 b of the resonators 3 and 4 areconnected with an internal electrode 65, extending continuously acrossthe top surface of the combination of the acceleration-sensor element 2Band the casing components 6, via the respective connection electrodes 34and 44. The internal electrodes 61 a and 61 b are respectively connectedwith the external electrodes 72 and 73 disposed on the outer surface ofthe cover components 7, whereas the internal electrode 65 is connectedwith the external electrode 71.

When using the acceleration-sensor element 2B of a double-supportedstructure as in the second preferred embodiment, signals can be detectedfrom both longitudinal ends of the acceleration-sensor element 2B. Sucha structure allows easier extraction of electrodes in comparison withthe acceleration-sensor element 2A of a cantilever structure. Forexample, the extraction electrode 5 b disposed on a side surface of thebase plate 5 and the extraction electrodes 62 a and 63 a disposed oninner side surfaces of the casing components 6 can be omitted.Furthermore, the anisotropic electrically-conductive adhesive providedfor connecting the connection electrodes 33 and 43 with the respectiveextraction electrodes 62 a and 63 a can also be omitted.

The acceleration sensor according to the present invention is notlimited to the above-described preferred embodiments.

For example, although the resonators 3 and 4 used in the first andsecond preferred embodiments are of a thickness-shear vibration type,resonators of other alternative vibration types (such as athickness-extensional vibration type or a longitudinal vibration type)may be used.

Furthermore, although the base plate and each of the first and secondresonators in the above-described preferred embodiments have a gaptherebetween given a wider dimension than the range in which theresonator vibrates in an enclosed manner, the base plate and eachresonator may alternatively be joined with each other in an opposingmanner such that the surfaces of the base plate and the resonator areentirely attached to each other. Such an entirely-attached state maycause deterioration of the performance (Q and K) of the resonators sincethe base plate limits the vibration of the resonators, but is effectivein view of the efficiency for generating stress in response to theacceleration.

In the above-described preferred embodiments, the electrodes 3 a and 3 bof the resonator 3 and the electrodes 4 a and 4 b of the resonator 4 areexposed at the upper and lower sides with respect to the verticaldirection (z-axis direction). Thus, the trimming process for theelectrodes can be performed from both upper and lower sides.Alternatively, the electrodes may be exposed only at one of the upperand lower sides.

If the electrodes of the resonators 3 and 4 are exposed only at one ofthe upper and lower sides, the extraction electrodes 62 b, 63 b, 64 tobe in contact with the terminals of a measuring device during thetrimming process for the electrodes are preferably exposed at the sameside as the electrodes of the resonators 3 and 4 in view of betterworkability for the trimming process.

Although the present invention has been described and illustrated indetail with reference to certain preferred embodiments thereof, it isclearly understood that the same is by way of illustration and exampleonly and is not to be taken by way of limitation, the spirit and scopeof the present invention being limited only by the terms of the appendedclaims.

1. An acceleration sensor comprising: a base plate; and first and secondresonators each including a piezoelectric material and each havingelectrodes on two opposite main surfaces thereof, each of the first andsecond resonators having a vibrating section at an intermediate portionof the respective resonator with respect to a longitudinal directionthereof; wherein the first and second resonators are attached toopposite sides of the base plate with respect to a direction in whichacceleration is applied so as to define a bimorph acceleration-sensorelement, at least one longitudinal end of the acceleration-sensorelement is fixed such that the first and second resonators bend in thesame direction in response to the acceleration, and changes in frequencyor changes in impedance in the first and second resonators caused by thebending of the acceleration-sensor element are differentially detectedin order to detect the acceleration; opposite sides of theacceleration-sensor element with respect to the application direction ofacceleration are respectively covered with a pair of casing components;and the first and second resonators are attached to the base plate suchthat the electrodes of the first and second resonators face at least oneof opposite open planes defined by a combination of theacceleration-sensor element and the casing components with respect to adirection that is substantially perpendicular to the applicationdirection of acceleration.
 2. The acceleration sensor according to claim1, wherein one of the opposite open planes defined by the combination ofthe acceleration-sensor element and the casing components with respectto the direction that is substantially perpendicular to the applicationdirection of acceleration is provided with a first electrode connectedwith one of the electrodes of the first resonator, a second electrodeconnected with one of the electrodes of the second resonator, and athird electrode connected with the other electrode of the firstresonator and with the other electrode of the second resonator.
 3. Theacceleration sensor according to claim 1, wherein the base plate and thefirst and second resonators are made of at least one material havingsubstantially the same coefficient of thermal expansion.
 4. Theacceleration sensor according to claim 1, wherein only one longitudinalend of the acceleration-sensor element is fixed, wherein the oppositeopen planes defined by the combination of the acceleration-sensorelement and the casing components with respect to the direction that issubstantially perpendicular to the application direction of accelerationare respectively covered with a pair of cover components such that adisplacement portion of the acceleration-sensor element, which isbendable in response to the acceleration, is disposed within an enclosedspace, one of the electrodes in each of the first and second resonatorsis disposed at a free-end side of the resonator and is connected with acommon electrode via an extraction electrode provided on the base plate,the common electrode being provided at a fixed-end side of an outersurface of a combination of the casing components and the covercomponents, the other electrode in the first resonator is disposed at abase-end side of the first resonator, the electrode being connected witha first independent electrode provided at a free-end side of the outersurface of the combination of the casing components and the covercomponents, the electrode being connected with the first independentelectrode via a first extraction electrode provided on one of the casingcomponents, and the other electrode in the second resonator is disposedat a base-end side of the second resonator, said electrode beingconnected with a second independent electrode provided at the free-endside of the outer surface of the combination of the casing componentsand the cover components, said electrode being connected with the secondindependent electrode via a second extraction electrode provided on theother casing component.
 5. The acceleration sensor according to claim 1,wherein a height of the first and second resonators in the directionthat is substantially perpendicular to the application direction ofacceleration is smaller than a height of the base plate in the directionthat is substantially perpendicular to the application direction ofacceleration.
 6. The acceleration sensor according to claim 5, whereinthe first and second resonators are attached to the opposite sides ofthe base plate at positions where the first and second resonators areopposed to each other.
 7. The acceleration sensor according to claim 6,wherein each of the first and second resonators is attached to a centralportion of the base plate with respect to a height direction of the baseplate, the height direction being substantially perpendicular to theapplication direction of acceleration.